Pikaia gracilens is an ancient, early chordate marine animal found in the Middle Cambrian Burgess Shale of British Columbia. Scientists first described it in 1911 as an annelid, but in 1979, researchers Harry B. Whittington and Simon Conway Morris reclassified it as a chordate. It is known as "the most famous early chordate fossil" or "the earliest described Cambrian chordate." It lived during the later stages of the Cambrian explosion, a time of rapid diversification of life. Over 100 specimens of Pikaia have been discovered since its initial discovery.
The body of Pikaia resembles that of a lancelet, and it may have swum like an eel. A notochord (a flexible rod-like structure) and myomeres (muscle segments) run the length of its body. These features are key traits of chordates. Its body is covered by a cuticle, a tough outer layer common in invertebrates and some early chordates. A 2024 study found evidence of a gut canal, a dorsal nerve cord, and myomeres, and suggested that earlier interpretations may have shown the fossil upside down.
Scientists are still studying Pikaia’s exact place in the evolutionary tree. Recent research suggests it is likely a primitive chordate with some traits of modern chordates. Earlier theories proposed it might be related to cephalochordates, craniates, or a chordate not closely linked to living species. It is often mistakenly called the ancestor of all vertebrates, the oldest fish, or the ancestor of humans. However, it is generally considered a basal chordate, closely related to vertebrate ancestors but not an ancestor itself.
Discovery
The fossils of Pikaia gracilens were discovered by Charles Walcott in the Burgess Shale member of the Stephen Formation in British Columbia. He described the fossils in 1911 and named the species after Pika Peak, a mountain in Alberta, Canada. Walcott noticed the regular body segmentation, a feature common in annelids, and classified Pikaia as a polychaete worm. He created a new family, Pikaidae, for it. Later, in 1962, Princeton paleontologist Benjamin Franklin Howell changed the family name to Pikaiidae. Walcott acknowledged that his classification had limitations, as he wrote: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of parapodia [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth."
In the 1970s, University of Cambridge paleontologist Harry B. Whittington and his student Simon Conway Morris re-examined the Burgess Shale fossils. They identified anatomical details of Pikaia that had not been noticed before, such as features resembling a notochord and muscle blocks, which are key structures in chordates, not annelids. In 1977, Conway Morris published a paper suggesting Pikaia might belong to the chordate group, though he did not provide detailed explanations. Whittington and Conway Morris later wrote in Scientific American in 1979:
"Finally, we find among the Burgess Shale fauna one of the earliest-known invertebrate representatives of our own conspicuous corner of the animal kingdom: the chordate phylum… The chordates are represented in the Burgess Shale by the genus Pikaia and the single species P. gracilens."
In 1979, Conway Morris formally classified P. gracilens as a chordate in a paper published in Annual Review of Ecology and Systematics. However, he did not use microscopic analysis to confirm the chordate features, so the classification was described as "putative" (tentative). For many years, scientists remained skeptical about Pikaia's chordate status. In 2012, Conway Morris and Jean-Bernard Caron published detailed analyses confirming Pikaia's place in the chordate group, leading to general acceptance of this classification.
The Pikaia fossils are found only in specific layers of rock exposed on Fossil Ridge, near Yoho National Park. From the same location, fossils of another Cambrian-era chordate, Metaspriggina, were discovered in 1993. These fossils are preserved in the Smithsonian Institution and the Royal Ontario Museum.
Description
Pikaia has a body shaped like a lancelet, which tapers at both ends and is flat from side to side. It does not have a clear head. On average, it is about 1 + 1⁄2 inches (3.8 cm) long. Walcott found some individuals that were up to 5 cm (2.0 in) long. Pikaia has two large, antenna-like tentacles on its head, similar to those of snails. These tentacles form a two-lobed structure on the head. The tentacles may be similar to those of modern hagfish, which are jawless chordates. Pikaia has a small, round mouth that could be used to eat tiny food particles in one bite. Small appendages are found on either side of the head, just below the mouth, but their purpose is unknown. The throat area has six pairs of slits with tiny filaments that may have helped with breathing. This is different from modern lancelets, which have clear gill slits used for filter feeding.
A key feature of Pikaia is its cuticle, a hard protein layer found in many invertebrates, such as arthropods, mollusks, echinoderms, and nematodes. Unlike typical cuticles, Pikaia’s cuticle does not provide hard external protection, and its body was mostly soft. Despite being primitive, Pikaia had traits important for vertebrates. When alive, it had a flat, leaf-shaped body with a wide tail fin. Its body was divided into pairs of muscle blocks, visible as faint vertical lines. These muscles were on either side of a flexible rod-like structure that ran from the head to the tail.
Pikaia was an active swimmer. It likely moved by making S-shaped, zigzag curves, similar to eels. Fish later inherited this swimming style but had stiffer backbones. These traits may have helped Pikaia filter small particles from water as it swam. Pikaia likely swam slowly because it lacked fast-twitch muscle fibers, which are needed for quick movement in modern chordates.
Walcott first described Pikaia as a long, thin animal that tapers at both ends. Its body had many segments marked by shiny, ring-like lines. The head was small with two large eyes and two tentacles. The first five segments behind the head had short, two-part structures called parapodia. Walcott described the digestive tract as running from one end of the body to the other. He thought Pikaia was a type of worm similar to annelids.
Later, Whittington and Conway Morris found that Walcott’s description was incorrect. They compared Pikaia’s body segments to living animals and found they were more like muscle bundles in chordates, such as Amphioxus (Branchiostoma), and not like annelid worms. They believed the muscles were important for swimming in wiggling motions. Walcott’s observation of the digestive tract was not a typical digestive system but ran alongside a stiff rod that resembled a notochord. In 1979, they concluded that Pikaia was not a worm but a chordate.
Conway Morris believed the stiff rod was a notochord and the segments were muscle blocks. He stated that Pikaia was a primitive chordate, not a worm, and possibly close to the ancestor of fish. In 1982, he noted Pikaia may have had one or more fins but did not specify their location.
Pikaia was not widely recognized as a chordate fossil until 1989. That year, Stephen Jay Gould wrote in his book Wonderful Life that Pikaia was not a worm but a chordate, part of the same group as humans. This helped people understand that Pikaia was a chordate and an ancestor of vertebrates.
In 1993, Conway Morris suggested Pikaia had structures that might be gill slits. He noted tiny holes on the side of the throat, which are usually gill slits in living chordates. He also found that Pikaia was similar to Amphioxus in most ways, except its notochord did not reach the front of the body.
Not all scientists agreed that Pikaia was a chordate. In 2001, Nicholas D. Holland and Junyuan Chen criticized Gould’s claim, saying the evidence for Pikaia being a chordate was weak. In 2010, some scientists argued Pikaia looked more like an invertebrate, similar to the Tully monster, which is still debated as a chordate or invertebrate.
Pikaia’s cuticle, a hard layer on its body, suggests it was an invertebrate, not a vertebrate. However, other early chordates, like Haikouichthys and Myllokunmingia, did not have cuticles, showing that cuticles are not needed for preservation. The presence of tentacles remains unclear, and Pikaia’s classification is still debated. Its body closely resembles the modern creature Branchiostoma.
In 1979, a fossil called Myoscolex ateles was found in Australia and debated as an ancient annelid. Jerzy Dzik noted that Myoscolex had a smooth cuticle and muscular segments like Pikaia, and its backside had structures similar to Pikaia’s tentacles. He suggested Myoscolex might be related to annelids rather than chordates.
In 2012, Conway Morris and Jean-Bernard Caron published a detailed study of Pikaia in Biological Reviews. They examined 114 fossils and confirmed Pikaia was a chordate. The fossils show features that define it as a primitive chordate. All Pikaia fossils are between 1.5 and 6 centimeters (0.59 to 2.36 inches) long, with an average length of 4 centimeters (1.6 inches). Its body was laterally compressed, meaning it was taller than it was wide.
Evolutionary importance
Scientists have debated whether Pikaia, a small fossil from the Cambrian period, is an ancestor of vertebrates, even though it looks like a worm. The fossil, found in the Burgess Shale, shows features of chordates, such as a notochord (a flexible rod-like structure along the back), a dorsal nerve cord, and muscle blocks on either side of the body. These features are important for the evolution of vertebrates.
The notochord helps the body bend side to side, allowing the animal to swim. In fish and other vertebrates, the notochord becomes the backbone, which supports the body, protects the nerve cord, and allows movement. A modern relative of Pikaia, the lancelet Branchiostoma, still exists today. Both Pikaia and lancelets belong to the chordate group, from which vertebrates evolved. However, recent studies suggest that tunicates, not lancelets, are the closest living relatives to vertebrates.
Pikaia had a cuticle, a type of outer layer found in some invertebrates. This feature is also seen in another Cambrian fossil, Myoscolex ateles, which had a smooth cuticle and muscle segments like Pikaia. While chordates usually lack a cuticle, some lower chordates, like cephalochordates, have a type of cuticle, showing that some ancient traits remain in these groups.
Scientists later reviewed earlier descriptions of Pikaia and found that some features are unique to it and not helpful in proving it is a very early chordate.
Chordates like Pikaia and Branchiostoma show the first signs of cephalization, or the development of a head. Scientists think this happened because these animals had long bodies, swam forward, and had mouths at the front to explore their environment. Structures for sensing the environment, such as those for seeing, feeling, and smelling, developed near the mouth. These structures sent information to a swelling in the nerve cord, which became the precursor to the brain.
Pikaia is one of the most important fossils found in the Burgess Shale, a famous site of Cambrian fossils. In 1979, scientists Whittington and Conway Morris recognized Pikaia as a chordate, showing that chordates evolved much earlier than previously thought. They noted that Pikaia’s well-preserved remains are a key example of the chordate group, which includes all vertebrates, including humans. However, Pikaia is often mistakenly called the ancestor of all vertebrates or the oldest fish, even though it is not the direct ancestor of humans.
Before Pikaia was studied, scientists believed chordates appeared later, during the Ordovician period. The discovery of Cambrian chordates, like Pikaia, changed scientific views about evolution. Stephen Jay Gould, a scientist, said that finding Pikaia showed that chordates evolved earlier than expected. However, he did not think Pikaia was the direct ancestor of vertebrates, as other fossils might be more closely related.
In 1988, Gould discussed Pikaia in a speech, saying that if Pikaia had not survived the mass extinction event known as the Cambrian "Burgess decimation," humans and other chordates might not exist today. He argued that evolutionary changes depend on chance events, a concept called evolutionary contingency. If Pikaia had not existed, the evolution of chordates and other life forms might have taken a different path. Gould’s ideas about contingency have influenced many scientific studies, showing that large-scale evolution is unpredictable and not guaranteed to follow a set path.
Ecology
Pikaia may have been an active swimming organism that moved near the ocean floor by wiggling its flat tail from side to side. Its front appendages were probably not used for eating but may have helped it breathe. Pikaia likely ate tiny bits of organic material, such as small microbes.